An integrated circuit (IC) is a combination of interconnected circuit elements inseparably associated on or within a continuous substrate, the substrate being the supporting material upon which or within which an integrated circuit is fabricated or to which an integrated circuit is attached. An integrated circuit is generally fabricated within a chip of semiconductor material, usually silicon, with the resistors, capacitors, diodes, transistors, etc. (as required) built into and/or on the chip. The semiconductor body is either single crystal material or single crystal islands in a polycrystalline material, depending on the method for electrical isolation of the circuit components.
Epitaxial silicon layers are routinely deposited on silicon substrates during the manufacture of film and hybrid integrated circuits. The deposition of high quality single-crystal silicon on an insulator, however, places rigid demands on the insulator. The selection of spinel and sapphire as film substrates are natural choices because they provide lattice planes that are close matches to those of silicon. However, irrespective of the substrate, such epitaxial silicon layers have limited applications by reason of their minority carrier lifetimes.
Silicon layers are epitaxially grown on sapphire substrates (silicon-on-sapphire) in the fabrication of high-speed integrated circuits. But such silicon layers are particularly limited in their applications by their very low minority carrier lifetimes (typically about 10 nanoseconds). See Allison, Dumin, Heiman, Mueller and Robinson, Proc. IEEE, 57, 1490 (1969). The application for such silicon layers has been mainly limited to enhancement-mode MOS transistors. However, even in this application the low minority carrier lifetime is deleterious to the electrical characteristics of the device. While the operation of enhancement-mode MOS transistors is not dependent upon minority carriers, the minority carrier lifetime is important in that it determines the leakage current of the reverse biased drain PN junction. This leakage current is especially important in complementary MOS transistor circuits, where the ultimate quiescent power dissipation is determined by the leakage current of the drain diode. The leakage current is inversely proportional to the minority carrier lifetime in the PN junction space-charge region.
Epitaxially grown silicon layers on substrates such as sapphire cannot be used in the making of bipolar transistors. Even though the base widths of such devices are usually very narrow, minority carrier lifetime is extremely important to the gain of bipolar transistors. Both hydrogen chloride (HCl) and chlorine (Cl.sub.2) injection have been used during oxidation to raise the lifetime of such devices into the 40-50 nanosecond range. See, Robinson and Heiman, J. Electrochem. Soc. 118, 141 (1971); and Ronen and Robinson, J. Electrochem. Soc. 119, 747 (1972). The mechanism responsible is believed to be a gettering action of metallic impurities through the formation of volatile metal chlorides. However, even 50 nanoseconds, which corresponds to a diffusion length of a few microns, is too low for the fabrication of bipolar transistors.
Despite all this, the ability to isolate components formed with epitaxially grown silicon layers by using the insulating properties of the substrate and the lower capacitance associated with PN junctions formed in such layers make such layers extremely desirable in integrated circuit fabrication. Such films would be generally used in integrated circuits if minority carrier lifetimes in the microsecond range could be achieved.